Power line carrier telephone system



POWER LINE; GAR-R DER TELEPHONE SYSTEM Filed Aug. 10, 1929 3Sheets-Stem: I

INVENTDE .J D. SARROS ATTORNEY Aug. 23, 1932. J. D. SARROS 1,373,334

POWER LINE CARRIER TELEPHONE SYSTEM Filed Aug. 10. 1929 s sheets-sheet s//v ENTDR Y J D. SAR'EDS ATTORNEY Patented Aug. 23, 1932 UNITED STATESPATENT OFFICE "JAMES D. SARROS, 0F LONG ISLAND CITY, NEW YORK, ASSIGNORTO BELL TELEPHONE LABORATORIES, INCORIPORATED, OF YORK, N. Y., ACORPORATION OF NEW YORK POWER LINE CARRIER TELEPHONE SYSTEM Applicationfiled August 10, 1929. Serial No. 384,876.

This invention relates to carrier current communication systems andparticularly to power line carrier current communication systems. a

One object of the invention is to provide an improved carrier currenttransmission system employing the same carrier wave for communication inopposite directions, transmitting a code signal of carrier waves forselectively calling a station and transmitting the carrier wave and bothside bands during communication between stations.

Another object of the invention is to provide a power line carriercurrent communication system having a signal station with a transmittingand a receiving channel connected to the power line that shall block thetransmitting channel when the station is idle and when the receivingchannel is in use and that shall block the receiving channelwhen thetransmitting channel is in use while communicating with or callinganother station.

A further object of the invention is to provide a power line carriercurrent communication system of the above indicated type that shallblock the transmitting channel at a station by impressing negativeblocking potential on atube in the transmitting channel when the stationis idle or the receiving channel is in service and that shall block thereceiving channel by impressing negative blocking potential on the gridof a tube in the receiving channel when the transmitting channel is inservice while communicating with orcalling another station.

The present invention is particularly adapted for use on high-tensionpower trans-' mission systems which transmit low-frequency high-voltagealternating current for power purposes. The signal currents are ofrelatively high frequency in order to facilitate 1 communication betweensignal stations and separation of the power current from the signalcurrents. V

In the patent to C. N. Nebel, No. 1,733,553 dated October 29, 1929, isdisclosed a power line carrier current system wherein communicationbetwen signal stations on a power line is effected in both directions bymeans of the same carrier wave and only one side band is transmittedduring communication. In the disclosed system one carrier wave is usedto effect communication in opposite directions between stations and thecarrier wave and both side bands are transmitted over the power lineduring communication b ween stations. Selective calling of a station iseflt'ected by transmitting a code signal of the carrier wave.

In'the system used to disclose the inventio a plurality of signalstations are connec ed to a high tension power line. The stations arecoupled to the power line by suitable coupling filters and preferably inthe manner vdisclosed in the patent to W. V. Wolfe, No. 1,678,133, datedJuly 24, 1928.

In the disclosed system a pluralityof carrier current signal stationsare connected to a high tension power line. Each signal stationcomprises a transmittingchannel, a receiving channel and duplex controlclrcuits for preventing simultaneous operation of the two channels. Theduplex control circuits are in many respects similar to the duplexcontrolcircuits disclosed in the patent to Charles N. Nebel, No.1,733,553. The transmitting and receiving channels are connected Theoscillator circuit is connected to the input circuit of the modulatortube by a, transformer having a potentiometer connected l0 across theprimary winding thereof to control the percentage of modulation andprevent overloading amplifiers in the system. The output circuit of themodulator is connected to two thermionic amplifier tubes whichare'connected in push-pull relationship. The two amplifier tubes areconnected through a power amplifier to the power line. The poweramplifier is similar in construc tion and operationto the poweramplifier disclosed in the above mentioned patent to Charles N. Nebel.

The receiving channel comprises a band pass filter which selects thecarrier wave and the two side bands used in this system and excludessignal waves which may be used by other signal systems connected to thepower line. The receiving bandpass filter is connected to a detectortube. The output circuit of the detector tube supplies voice frequencycurrents to amplifier tubes in push-pull relationship and filters totheoperators telephone set. The detector output circuit also suppliescarrier waves to a circuit for operating a selector. The grids of thedetector tube and the two amplifier tubes in push-pullrelationshipadjacent thereto are controlled by duplex control circuitsfor blocking the receiving channel when the transmitting channel is inservice. The duplex control circuits also govern the negative potentialimpressed on the grids of the modulator tube and the two amplifier tubesadjacent the modulator tube in the transmitting channel to block thetransmitting channel when the station is idle and when the receivingchannel 1s in service.

I The selective calling of a distant station is effected by transmittinga code signal of the carrier Wave over the power line to the distantstation. The operation of the selector key at the sending stationoperates the duplex control circuit so as to block the receiving channeland prevent blocking of the transmitting channel and transmits a codesignal of carrier waves from the oscillator to the distant station beingcalled.

The code signal of carrier waves received at the distant called stationoperates a suitable selector for indicating to the operator that thestation 'is being called. Means are provided whereby theselector'willonly be operated when carrier current free from modulatingcurrents is received. Inthe disclosed system a differential relay isprovided for insuring against operation of the selector duringcommunication or talking betw-een two stations. The difierential relayhas one winding-"energized by the detected voice frequency currents fromthe receiving channel and a second winding energized by rectifiedcarrier'waves from the receiving channel. Ifcarrier waves alone arereceived the relay will be operated to effect operation 35 of theselector. However, if carrier waves signal station constructed inaccordance with the invention.

Referring to the drawings, a signal station comprisinga transmittingchannel 1 and a receiving channel2 is connected by coupling means 3 to apower line comprising conductors 4 and 5. The boupling means between thesignal station and the\.power linesmay be of any suitable character andas before set forth is preferably of thetype disclosed in the patent toW. V. Wolfe, No. 1,678,133, dated July 24, 1928. A hybrid coil 6 pro-'vided with balancing impedance 7 connects the transmitting channel 1 andthe receiving channel 2 to an operators telephone set '8 comprising areceiver 9 and a transmitter 10.

The transmitting channel 1 comprises a three-element thermionicmodulator tube 11, which is connected to two thermionic amplifier tubes12 and 13 by means of a transformer 14. The amplifier tubes 12 and 13are connected together in push-pull relationship and have their outputcircuits connected to a power amplifier 15 by means of a transformer 16.A transformer 17 connects the input circuit of the modulator tube 11 tothe hybrid coil 6. An oscillation generator 18;, which is referably ofthe thermionic type, is connecte to the input circuit of the modulatortube 11 by means of transformer 19. A potentiometer 20 is connectedacross the primary winding of the transformer 19 for adjusting thepercentage of modulation to such .value that efficient transmission ofthe carrier wave and the two side bands can be obtained.

The power amplifier 15 included in the transmitting channel .1 comprisesa thermionic amplifier tube 21 and two thermionic amplifier tubes 22 and23 in push-pull relationship with respect to each other. The outputcircuit of the amplifier tube 21 is connected to the input circuits oftubes 22 and 23 by means of transformer 24 and the output circuits ofthe tubes 22 and 23 are connected to the coupling means 3 by means of atransformer 25.

Rectifying means 26 is provided for supplying grid biasing potential tothe tubes 21, 22 and 23 of the power amplifier and rectifying means 27is provided for supplying plate potential to the thermionic tubes 21, 22and 23 of the power amplifier.

The rectifying means 26, which supplies grid biasing potential to thetubes of the power amplifier, comprises a thermionic tube 28 which isconnected by a transformer 29 34 and a second armature 36 to control therectifying means 26 and 27. The relay controls the grid biasingpotential and the plate potential supplied to the tubes of the poweramplifier 15. The circuit of the rectifier tube 28 may be traced fromthe tube through the secondary winding 32 of transformer 29, retard coil39 and potentiometer sections 40, 41 and 42 to the plate of the tube 28.Condensers 43 and 44 are provided for smoothing out ripples in therectified current. a

The grid biasing circuit for the power amplifier tube 21 extends fromthe grid of the tube 21 through the secondary winding of transformer 16,conductor 45, section of the potentiometer and the secondary windings ofa transformer 46 to the filament of the tube 21. The grid biasingcircuit for the two amplifier tubes 22 and 23 in pushpull relationshipextends from the grids of the two tubes 22 and 23 through the secondarywindings of the transformer 24, conductor 47, sections 40 and 41 of thepotentiometer and the secondary windings ofthe transformer 46 to thefilaments of the tubes 22 and 23. a

The filaments of the power amplifier tubes 21, 22 and 23 are heated byalternating current from the source 30 supplied to the transformer 46.When the signal station is idle and relay 35 is released, the primarywinding of transformer 46 is connected to the source of alternatingcurrent 30 through a resistance element 48. The resistance element 48serves to limit the current supplied to the filaments of the tubes 21,22 and 23 in the power amplifier when the signal station is not in use.Upon operation of relay 35 the armature 36 short-circuits the resistanceelement 48 to insure heating the filaments of the power amplifier tubesto normal value.

The means 27 for supplying plate potential to the tubes 21, 22 and 23 ofthe power amplifier comprises two two-element thermionic rectifier tubes50 and 51 which are connected to the source of alternating current 30 byI ineans of a transformer 52. The transformer 52 is connected to thesource 30 through the armature 34 of the relay 35 to insure againstsupplying plate potential to the tubes of the power amplifier exceptwhen the station is in use. A transformer. 53 is provided for supplyingheating current from the source 30 to the filaments of the tubes 50 and51. Transformer 53 is connected to the source 30 through the armature 34of the relay 35 in order to insure-against heating the fila- 'ments ofthe rectifier tubes when the station is not in service.

The plate circuit for the thermionic tube 21 may be traced from theplate of the tube 21 through the primary winding of the transformer 24,conductor 54, retard coil 55, secondary windings of the transformer53,-rectifier tubes 50 and 51 in parallel, secondary windings of thetransformer 52 and secondary windings of the transformer 46 to thefilament of the tube 21 The circuit for supplying plate potential to thetubes 22 and 23 may be traced in 'a like manner.

A battery 56 is provided for supplying grid biasing potential andfilament current to the tubes 11, 12 and 13 in the transmittingchannel 1. This battery also supplies grid biasing potential andfilament current to many other thermionic tubes in the system. Asuitable charger or rectifier 57, which is connected to a source ofalternating current 58, is controlled by a relay 59 for charging thebattery 56. The relay 59 which is provided with two armatures 60 and 61is operated in parallel with the relay 35 in a manner to be more clearlyset forth hereinafter. When the relay 59 is released and the signalstation is idle the battery 56 is charged by a circuit including thearmature 60. Gridbiasing potential and filament current is supplied to aconductor 63 at this time through a resistance element 62 and resistanceelement 64.

Upon operation of the relay 59, the armature 60 disconnects the chargingmeans connected to the battery 56 and excludes the re sistance element62 from the battery circuit. The filament heating circuit for thethermionic tubes 11, 12 and 13 may be traced from the negative terminalof the battery 56 through armature 60, resistance element 64, conductor63, conductor 49, resistance element 65, filaments of the tubes 12 and13 in parallel, filament' of the thermionic tube 11 and ground returntothe ositive terminal of the battery 56. A grid b-iasingcircuit for thethermionic tubes 11, 12, and 13 in the transmitting channel will bedescribed when reference is made to the circuits for efi'ectingduplexing operation.

Plate potential for the tubes 11, 12 and 13 in the transmitting channeland for many other tubes in the signal station is provided by therectifying means comprising thermionic tubes 66, 67, 68 and 69 connectedin parallel relationship. Transformers 70 and 71 are provided forconnecting the rectifying tubes 66 to 69, inclusive, toth e'source ofalternating current 36. "The secondary wind-- ings of the transformer 71supply heating 69, inclusive, and the secondary windings of thetransformer 70 supply plate potential to the rectifying tubes 66 to 69,inclusive. Voltage compensating means 72 of the type shown and describedin the above mentioned patent to C. N. Nebel, No. 1,733,553 is providedin circuit with the rectifying tubes 66 to 69, inclusive.

The plate circuit for the modulator tube 11 in the transmitting channelmay be traced from the filaments of the rectifier tubes 66 to 69,inclusive, through the secondary windings of the transformers 70 and 71,voltage compensating device 72, retard coil 73, conductor 74, retardcoil 75, primary winding of transformer 14, plate and filament of themodulator tube 11 and ground return to the rectifier tubes 66 to 69,inclusive. In a like manner the plate circuit for the amplifier tubes 12and 13 may be traced.

The receiving channel 2 comprises a band pass filter 76- which isconnected to the coupling means 3 through a potentiometer 77. The bandpass filter 76 selects the carrier wave and the side bands which arecharacteristic to the system under consideration- It is to be understoodthat other carrier current systems employing different carrier waves maybe connected to the same power lines 4 and 5. The band ass filter 76 isconnected through a trans ormer 78 to a three-element thermionicdetector tube 79. The output circuit of the detector tube 79 isconnected by a transformer 80 to two amplifier tubes 81 and 82 which areconnected in push-pull relationship. The amplifier tubes 81 and 82 areconnected by a transformer 83, a low pass filter 84 and atransformer 85to a voice frequency amplifier tube 86. The voice frequency amplifiertube 86 is connected by the hybrid coil 6 to the op-' erators set 8. Thetransformer 85 is also connectedto an amplifier tube 87 which in turn isconnected by a transformer 88 to a thermionic rectifier tube 89. Thethermionic rectifier tube 89 governs the duplex control circuits 180when the receiving channel is in use for insuring the blocking of themodulator 11 and amplifier tubes 12 and 13 in the transmitting channel.

The output circuit of the detector tube 79 is not only connected to theamplifier tubes 81 and 82 by the transformer but is connecteclby atransformer 90 to a thermionic amplifier tube 91. The thermionicamplifier tube 91 is connected by transformer 92 to a rectifier tube 93.The transformer 90 in the output circuit of detector 79 supplies 'high'frequency carrier current which may pass through the detector tube tothe amplifier tube 91 and rectifier tube 93. The carrier currentamplified by the tube 91 and rectified by the tube'93 is used to controlthe operation of a differential relayf 94.

the receiving channel is supplied by the rectifying tubes 66 to 69inclusive. Filament current and grid biasing potential for the tubes inthe receiving channel is supplied by bat tery 56. The grid biasingcircuits for the tubes in the receiving channel .utilized in blockingsaid channel when the transmitting channel is in use will be describedwhen reference is made to the duplex control circuits.

The circuit from the rectifier to the plate of the detector tube 79 maybe traced from the filaments of the rectifier tubes 66 to 69, inclusive,through secondary windings of the transformer 70, voltage compensator72, impedance coil 73, conductor 74, conductor 95, .retard coil 96,primary winding of transformer 80, primary winding of transformer 90,plate and filament of the detector tube 79 and a ground return conductor97 'to the rectifying tubes 66 to 69 inclusive. In a like mannerpotential for the plates of the tubes 81 and 82 and 91 is suppliedfromthe conductor 95 connected to the rectifying tubes 66 to 69, inclusive.Plate potential for'the thermionic tubes 86 and 87 is supplied from aconductor 97a connected to the rectifying tubes 66,130 69, inclusive,.through the armature 61 of relay 59. The plate circuit for therectifying tube 93 will be traced when reference is made to theselective calling circuits for the signal station.

The filament circuit for the tubes .81, 82, 93 and 79 may be traced fromthe conductor 63, which is connected to the negative terminal of thebattery 56, through a resistance element 99, filaments of tubes 81 and82 in parallel, resistance element 100, the filaments of the tubes 93and 79 in parallel ind a ground conductor 97 to the positive terminal ofthebattery 56. The filament circuit for the tube 91 extends from theconductor 63 through resistance elements 101 and 102. The filament.circuit of the tubes 86, 87, 89 anda tube 103 in the duplex controlcircuits extends from the conductor 63 through resistance element 104,and filaments of the tubes 87, 86, 89 and 103 to a ground conductor 105which is connected to the positive terminal of the battery 56.

The duplex control circuits which govern the blocking of tubes in thetransmitting channel when the receiving channel is in service and theblockingof tubes in the receiving 1 potential to the tube 114 alsosupplies oura transformer 111 to a rectifier tube 112. The circuit ofthe rectifier tube 112 includes a resistance element 113 which governsthe negative biasing potential impressed on the detector tube 79 and thetwo amplifier tubes 81 and 82. A three-element thermionic tube -114 isprovided having a resistance element 115 in the output circuit thereoffor governing the negative biasing potential impressed on the grid ofthe modulator tube 11 and the amplifier tubes 12 and 13 in thetransmitting channel. The potential impressed on the grid of the tube114 is controlled by the potential across the resistance element 113.

Plate potential for the tube 114 is obtained from a potentiometer'117 inthe output circuit of a rectifiertube 118. The output circuit for thetube 114 may be traced from the filament of the tube through theresistance element 115, conductor 119, a portion of the potentiometer11; and conductor 120 to the plate of the tube 11 The rectifier tube 118which supplies plate rent for operating a selector 121. A transformer122 is provided for connecting the rectifier tube to the source ofalternating current 30. One secondary winding of thetransformer 122supplies filament current to the rectifier tube and another secondarywinding is connected in the plate circuit of the rectifier tube. A thirdsecondary Winding of the transformer 122 is connected to a conductor 123for supplying a 60 cycle signal tone to' the duplex control circuitsduringtransmission of a calling signal. Suitable filteringmeans isprovided between the rectifier tube 118 and the potentiometer 117.

A resistance element 125 in the output circuit of the rectifier tube 89controls the negative grid bias on the tubes 103 and 106 in the duplexcontrol circuits. The rectifier tube 89 impresses potential across theresistance element 125 when the receiving channel is in service. Thepotential produced across the resistance element 125 impresses a strongnegative potential on the grids of the two tubes 103 and 106.Accordingly, when the tubes 103 and 106 are blocked no power isamplified by the tube 110 for supplying current to the rectifier tube112. Consequently substantially no potential is impressed across theresistance element 113 and substantially no negative biasing potentialis impressed on the grid of the tube 114. When no negative biasingpotential is impressed on the grid of the tube 114, a relatively strongflow of current through the tube 114 and the resistance element 115 isproduced. It may be noted that when the station is idle no potential isimpressed across resistance element 113 and a flow of current iseifected through tube 114 and resistance 115. Negative potential on thegrids of the modulator tube 11 and the amplifier tubes 12 and 13 in thetransmitting channel varies according to the potential impressed acrossthe resistance element 115. Accordingly the transmitting channel isblocked not only when the receiving channel is in use, but also when thestation is idle.

The grid biasing circuit for the modulator tube 11 may be traced fromthe grid of the modulator tube 11 through the primary winding of thetransformer 19, secondary winding of the transformer 17, conductor 119,resistance element 115, and filaments of the tubes 13 and 12 and groundto the filament of the modulator tube 11. In like manner, the gridcircuit for the amplifier tubes 12 and 13 may be traced.

The negative grid potential for the detector tube 79 and the amplifiertubes 81 and 82 in the receiving channel 2 is obtained from theresistance element 113 in circuit with the rectifier tube 112 of. theduplex control circuits 180. The grid circuit for tubes 81 and 82 may betraced from one terminal of the resistance element 113' through aresistance element 124, and secondary windings of transformer to thegrids of the tubes 81 and 82. The filaments of the tubes 81 and 82 areconnected to the other terminal of resistance element 113 by conductor63. grid circuit for the detector tube 79 extends from one terminal ofthe resistance element 113 through a resistance element 126, secondarywinding of the transformer 78 to the grid of the detecctor tube 79,filament of the detector tube 7 9, resistance element 100, filament ofthe tube 81, resistance element 99, and the conductor 63 to the otherterminal of the resistance element 113.

A relay 127 is provided for impressing a 60 cycle signal tone on theduplex control circuits 180 from the transformer 122 when selectivecalling of the distant station is being efi'ected. The signal tone isimpressed upon the duplex control circuits at this time in order toinsure blocking of the receiving channel. A signal pulsing relay 128provided with armatures 129 and 130 is under the control of a selector,key 131 when selectively calling a distant station. The armature 129controls a slow releasing relay 132 and the armature 130 controls ashunt circuit across the transmitting channel to govern the impulses ofunmodulated carrier current transmitted to the power line. The slow releasing relay 132 is provided With an armature 133 for completing acircuit through the energizing coil of the relay 127. The relay 132being slow releasing serves to maintain the relay 127 operated and asignal tone on the duplex control circuits whenever the relay 128 isbeing intermittently operated by the selector key 131. p

A relay 134 is controlled by the receiver switchhook and is providedwith an armature 135 for completing a circuit to energize the relays 35'and 59. The relay 134 is pro- The vided with a second armature 136 forpreventing operation of the differential relay 94 except by the selectorkey when the receiver 9 is removed from the switchhook. The circuit forenergizing the relays 35 and 59 may be traced from the battery supplyconductor 63 through a battery supply conductor 137,

armature 135, conductor 139, and coils of the relays 59 and in parallelto the positive terminal of the battery 56.

The differential relay 94 which operates the selector 121 comprisesthree windings 140,

141 and 142. The winding 140 is energizedin accordance with theoperation of the recti- -fier tube 93 in the receiving circuit. Therec-- when carrier current modulated with voice frequency current istransmitted over the power line. The third winding 142 of thedifferential relay is operated by the selector key 131 and the selectorkey (not shown) at the extenslon station. The armature 145 of thedifferential relay operates between contact members 146 and 147. Thecontact members 146 and 147 are connected to opposite terminals of thepotentiometer 117 and the armature 145 is connected through theoperating coil of the selector to a mid-point of the potentiometer 117.The windings 140 and 141 oppose each other so that when both windingsare energized no operation of the selector can be effected. Ifunmodulated carrier current impulses are received the winding 140 isenergized for effecting operation of the selector. If modulated carriercurrent impulses are received, it is apparent both windings 140 and 141are energized to prevent operation ofthe selector. Preferably thedifferential relay 94 is polarized.

Uommunicatye'pn with a distant station In order to communicate with adistant station the receiver is removed from {the switchhook and acircuit completed from the battery conductor 137 for operating the relay134. The relay 134 efi'ects operatlon of the relays 35 and 59. The relay35 effects operation of the power supply means for the power amplifiertubes 21, 22. and 23 in the transmitting. channel. The relay 59disconnects the charger from the battery 56 and supplies plate potentialto the thermionic tubes 110, 103 and 106 in the duplex control circuitsand to'the thermionic tubes 86 and 87 in the receiver channel. When thetransmitting channel is in service, voice frequency currents from thetransmitter 10 are transmitted through the hybrid coil 6 and thetransformer 17 to the input circuit of the modulator tube 11 to bemodulated with carrier current from the oscillator 18. The modulatedcurrents are amplified by the tubes 12 and 13 and the power amplifier 15before being transmited over the power line.

At this time the duplex control circuits which are connected to thetransmitting channel by the transformer 107 apply a relatively largepotential across the resistance element 113 in the circuit of therectifier 112. The potential across the resistance element 113 impressesa relatively large negative bias on the grid of the tube 114 to preventimpressing potential across the resistance element 115. Accordingly nonegative blocking potential at this time can be impressed on the gridsof the modulator tube 11 and the amplifier tubes 12 and 13. Theresistance element also impressed a blocking negative potential on thegrids of the detector tube 79 and the amplifier tubes 81 and 82 in thereceiving channel 2.

When the receiving channel is in service, detector tube 79 is operatedfor demodulating the carrier current received over the power line. Thevoice frequency currents in the out-put circuit of the detector 79 areamplified by the tubes 81 and 82 and passed through the filter 84. Thevoice frequency currents then pass through the amplifier 86 and thehybrid coil 6 to the operators telephone receiver 9. The circuits foroperating the telephone set receive power from the battery conductor 137and such circuits are completed through the ground conductor 160. Thevoice frequency currents in the receiving channel operate the amplifiertube 87 and the I rectifier tube 89 for impressing a potential acrossthe resistance element 125. The potential across the resistance element125 impresses a strong negative bias on the grids of the tubes 103 and106 in the duplex control circuits. Consequently no potential can beimpressed across the resistance element 113 and no negative bias can beimpressed on the grid of the tube 114. The potential impressed acrossthe resistance element 115 fromv the potentiometer 117 impresses astrong negative bias on the modulator tube 11 and amplifier tubes 12 and13 to block the transmitting channel.

Selectioely calling a distant station gizing the relay 132 extends fromsupply conductor 95 through the coil of the relay, and

.armature 129 to the ground conductor 160.

circuit across the input circuit of the power amplifier and thuscontrols the carrier current impulses transmitted over the power line.

' Gall received from a distant station If a selective calling signal isreceived from a distant station, the coil 140 on the difierential relay94 is operated by the amplifier tube 91 and the rectifier tube 93 inaccordance with the impulses of unmodulated carried current in theoutput circuitoof the detector tube 79.

Inasmuch as no modulated impulses of carrier current are being receivedat this time the winding 141 of the differential relay is deenergizedand operation of the selector 121 is effected by the differential relayin accordance with the code of received signals.

It is to be understood'that if so desired, an extension station of thetype disclosed in the patent to H. B. Arnold, No. 1,717,083 dated June11, 1929, may be connected to the selector 121, the relays 128 and 134and the hybrid coil 6. The conductors for connection to the extensioncircuit have been indicated in Figs. 1 and 2 of the drawings.

Modifications of the system and in the arrangement and location of partsmay be made within the spirit and scope of the invention, and suchmodifications are intended to be covered by the appended claims.

What is claimed is:

1. In a power line carrier current communication system, employing thesame carrier wave for transmission in both directions and'transmittingthe carrier wave and both side'bands during communication betweenstations, a power line, a signal statlon'having a transmitting and areceiving .channel connected to the power line, a thermionic modulatortube and two thermionic amplifier tubes in push-pull relationship insaid transmitting circuit, a detector tube and two thermionic amplifiertubes in push-pull relationship in said receiving channel, means 1 thegrids of the modulator tube and-the two amplifier tubes in thetransmitting circuit when the station is idle and when the receivingchannel is in use, and means for applying negative blocking potential tothe grids of the detector tube and two amplifier tubes in the receivingchannel when the transmitting channel is in use. I

2. In a power line carrier current communication' system employing thesame carrier wave for transmission in bothdirections and transmittingthe carrier wave and both side hands during communication betweenstations, a power line, a signal station having a transmitting channeland a receiving channel connected to the power line, an operatorstelephone set, a hybrid coil for connecting said telephone set to thetransmitting and receiving channels, a three-element thermionicmodulator tube and two three-element amplifier tubes in push-pullrelation in said transmitting channel, an oscillator connected to theinput circuit of said modulator tube, a

three-element thermionic detector tube and two three-element thermionicamplifier tubes in push-pull relation in said receiving channel, meansat said station including said tubes in the transmitting and receivingchannels to send and receive over the power lines the side bands and theunmodulated carrier component for communication purposes, means forimpressing a negative blocking potential on the grids of said modulatorand amplifier tubes in the transmitting channel when the receivingchannel is in use and for impressing a negative blocking potential 'onthe grids of said tubes in the receiving channel when the transmittingchannel is in use and means to transmit a code signal of unmodulatedcarrier current over the powe r line for calling three-elementthermionic modulator tube and two three-element thermionic amplifiertubes in push-pull relation in said transmitting circuit, athree-element thermionic detector tube and two three-element thermionicamplifier tubes in push-pull relation in said receiving channel, anoperators telephone set, a hybrid coil for connecting said telephone setto said transmitting channel and said receiving channel, means forimpressing a negative blocking potential on the grids of said tubes inthe transmitting channel when the receiving channel is in use and forimpressing a negative blocking potential on the grids of said tubes inthe receiving channel when the transmitting channel is in use to calland to communicate with a distant station, and means to transmit a codesignal of unmodulated carrier current over the calling purposes.

4. In a carrier current communication sys tem, a transmission line, asignal station connected to said line, a transmitting channel at powerline for said station comprising a three-element modulator tube, a highfrequency oscillator and two three-element amplifier tubes in pushpullrelationship, a receiving channel at said station comprising athree-element detector tube and two three-element amplifier tubes'in'push-pull relationship, means for impressin a negative blockingpotential on the grids o the modulator and amplifier tubes in saidtransmitting channel when the station is idle or the receiving channelis in service and for' impressing a negative blocking potential on thegrids of the tubes in said receiving channel when the transmittingchannel is in service, and means to transmit a code signal ofunmodulated carrier current for calling purposes.

In Witness whereof, I hereunto subscribe my name this 6th day of August,1929.

. JAMES D. SARROS.

